1. Field of the Invention
The present invention relates to a surface acoustic wave matched filter comprising a substrate, which includes a piezoelectric material. An input side electrode is provided on a surface of the substrate for converting a received input spread spectrum signal into a surface acoustic wave. An output side electrode is provided on a surface of the substrate for receiving a surface acoustic wave propagated from the input side electrode. The output side electrode is constructed to constitute a tapped delay line for producing a demodulated output signal.
2. Related Art Statement
The above mentioned surface acoustic wave filter, also called a correlator, has been widely used for demodulating an input spread spectrum signal in a spread spectrum communication. FIGS. 1A-1E are schematic views for explaining the general operation of such a spread spectrum communication. FIG. 1A illustrates a base band signal to be transmitted, and a data rate of the base band signal is denoted by f.sub.2 (bits/sec). The base band signal is then converted into a PN code having a code length N=5. In this example, a code "1" is denoted by "11001" and a code "0" by "00110" as depicted in FIG. 1B. The PN code has a chip rate of f.sub.1. By modulating a carrier having a frequency f.sub.0 with the PN code, a spread spectrum signal shown in FIG. 1C can be obtained. Therefore, the spread spectrum signal has a center frequency of f.sub.0.
The obtained spread spectrum signal is transmitted over a transmission path and is supplied to an input side electrode 2 of a surface acoustic wave matched filter 1 as shown in FIG. 1D. The input side electrode 2 converts the received spread spectrum signal into a surface acoustic wave. The surface acoustic wave matched filter 1 further comprises an output side electrode 3 from which a demodulated signal may be obtained. The output side electrode 3 is constructed by a tapped delay line including electrode fingers having a period .lambda. of v/f.sub.0, a tap distance L=v/f.sub.1 and a tap pattern corresponding to the polarity of the PN code, wherein v is a velocity of the surface acoustic wave propagating along the surface of a substrate 1 of the surface acoustic wave matched filter.
In the known surface acoustic wave matched filter, the output side electrode 3 constituting the tapped delay line is designed such that the tap distance L is equal to v/f.sub.1. It should be noted that the velocity v of the surface acoustic wave is a phase velocity. The phase velocity may be defined as a velocity at which a sinusoidal wave having a single frequency propagates along the surface of the substrate 1. In the known surface acoustic wave matched filter, the synchronization between the output side electrode 3 and the input spread spectrum signal can be difficult to attain, which causes a correlation peak level of the output signal to be decreased.
The inventors have conducted various experiments and analyses for the above mentioned decrease in the correlation peak level and have found that the tap distance of the matched filter is designed on the basis of the phase velocity of the surface acoustic wave. Recently, the substrate of the surface acoustic wave matched filter has been made of various materials. In order to make the operation frequency much higher, a laminated type substrate has been developed. For instance, the substrate is composed of a main body made of a material such as sapphire and diamond having a higher velocity for the surface acoustic wave and a thin film applied on the main body and made of a piezoelectric material such as aluminum nitride and zinc oxide. Upon use of such a thin film type substrate, a decrease in the correlation peak level has been remarkably observed.
The above mentioned decrease in the correlation peak level occurs because the known substrate made of a single crystal is substantially non-dispersive, but the thin film laminated type substrate has a dispersive characteristic which depends on film thickness. In this type of substrate revealing the dispersity, the surface acoustic wave propagates along the surface of the substrate predominantly at the group velocity instead of phase velocity. Therefore, when a surface acoustic wave matched filter having a thin film laminated type substrate is designed under a phase velocity, the spread spectrum signal cannot be detected in a synchronous manner by the output side electrode, which, in turn, causes a decrease in the correlation peak level.
The above mentioned problem of the decrease in the correlation peak level, due to the fact that the tap distance of the surface acoustic wave matched filter and the spread spectrum signal are not synchronized with each other, is not inherent to thin film laminated type substrates, but may equally occur in any kind of a substrate having such dispersity.
When the correlation peak level in the output demodulated signal is decreased, not only is the S/N of the output demodulated signal decreased, but there is also the requirement of a rather complicated and expensive circuit for processing the output demodulated signal having a low S/N. Consequently, a communication device having such a surface acoustic wave matched filter installed therein requires a large power consumption. This is particularly disadvantageous for portable type communication devices such as portable type telephone hand sets. Here, it should be noted that the S/N of the output demodulated signal from the surface acoustic wave matched filter is defined by a ratio of the correlation peak value of the matched filter output to a maximum side lobe peak value of output other than correlation peak.